Sheets and films comprising a polycarbonate resin as a substrate are excellent in light weight, transparency, heat resistance and impact resistance, and therefore are widely used as structural materials in place of glass. Recently, these sheets and films have also been used as decorative films because they are easy to print and thermoform. For example, these sheets and films are widely used for automobile applications (e.g., instrument covers, glazing, lamp lenses), for OA and electrical/electric applications (e.g., mobile phones, casings for mobile portable terminals, display panels), for building material applications (e.g., greenhouse covering materials, arcades, daylighting roofing materials), for road material applications (e.g., sidewalk wainscots, expressway fences), for industrial material applications (e.g., name plates), etc.
However, these sheets and films are of limited use due to their insufficient scratch resistance.
On the other hand, Patent Document 1 discloses a decorative film in which a polycarbonate-based resin layer is laminated with a thermoplastic acrylic resin layer to improve the scratch resistance of polycarbonate.
However, this approach is designed to laminate a thermoplastic acrylic resin and therefore ensures pencil hardness as high as that of the thermoplastic acrylic resin, but does not achieve sufficient improvement in scratch resistance (steel wool hardness) upon wiping off dust and/or sand deposited on the molded articles.
In addition, Patent Document 2 discloses a laminate obtained as follows: a sheet comprising a polycarbonate resin layer and a thermoplastic acrylic resin layer laminated thereon is placed within an injection molding machine die, a polycarbonate resin is injected into the die, and a coating composition containing colloidal silica having a particle size of 10 to 20 nm is applied and thermally cured over the surface of the resulting molded article.
This approach allows improvement in scratch resistance, but tends to result in poor appearance (e.g., dust adhesion, uneven application) and leads to low productivity because the coating composition is applied onto the molded article having a curved surface. Furthermore, this approach has a problem in that application of the coating composition causes an increase in haze. If the coating composition is applied onto the laminated sheet before being provided for injection molding, the productivity will be increased but there is a drawback in that cracks are likely to occur during forming and formable shapes are therefore limited to those with small curvature because the coating composition is less thermoplastic and hence is brittle.
Patent Document 2 states that silica in colloidal silica preferably has an average particle size of 4 to 20 nm and the silica content is preferably 50 to 200 parts by weight relative to organoalkoxysilane which is a member constituting the coating. In addition, laminates having a haze of 0.7% or higher are illustrated in the Example section.
As a result of extensive and intensive efforts, the inventors of the present invention have found that when silicon dioxide particles having an average particle size of 0.1 to 2 μm are used in a limited range of 0.1% to 1% by weight, relative to the total weight of the thermoplastic acrylic resin layer, it is possible to achieve excellent transparency and excellent scratch resistance, as well as excellent thermoformability.